Why we sleep: A theoretical view of sleep function

We propose that sleep begins within small groups of highly interconnected neurons and is characterized by altered input → output (i→o) relationships for any specific neuronal group. Further, experimental findings suggest that growth factors, released locally in response to neuronal activity, and acting in paracrine and autocrine fashions, induce the altered i→o relationships. These growth factors also act to provide the structural basis for synapses. Thus, we envision that sleep mechanisms (neural use- dependent induction of growth factors and their subsequent effects on i→o relationships) cannot be separated from sleep function (growth factor- induced synaptic sculpturing). This mechanism/function is envisioned to take place in all areas of the brain, including sleep regulatory circuits as well as throughout the cortex. Finally, the 'sleep' of neuronal groups (altered i→o relationships) is coordinated by the known sleep regulatory circuits and activational-projection systems in the brain. The theory extends and integrates existing sleep theories to cover a broader range of phenomena.